Formal:
* 절친 (jeolchin): This literally means "close friend" but is often used to refer to a best friend, especially in a formal setting.
* 단짝 (danjjak): This means "partner" and can be used to describe a best friend, especially someone you've been close with for a long time.
Informal:
* 베프 (bepeu): This is a shortened form of the English "best friend" and is very common among young people.
* 소울메이트 (soulmate): This is a direct translation of the English term and is used to describe a best friend who is like a soulmate.
* 친구 (chingu) + other words: You can also use the word "친구" (friend) along with other words to express the level of closeness, such as:
* 절친한 친구 (jeolchinhan chingu): very close friend
* 소중한 친구 (sojunghan chingu): precious friend
* 평생 친구 (pyeongsaeng chingu): lifelong friend
Ultimately, the best word to use depends on the specific situation and your relationship with the person.